Re: Expected warning maybe-uninitialized does not appear using g++13.2.0?

[Marc Glisse <marc.glisse@inria.fr>]

On Thu, 21 Dec 2023, David Malcolm via Gcc wrote:

> On Wed, 2023-12-20 at 11:16 -0800, Eric Batchelor wrote:
>> Hello, I unintentionally stumbled upon some strange behaviour that
>> occurred due to a typo.
>> I reproduced the behaviour where an object (std::string in my case)
>> can
>> be passed to a function by reference, uninitialized, WITHOUT a
>> compiler
>> warning.
>> Changing the code to pass the object by value DOES emit the warning.
>> I don't think the compiled code is incorrect, it segfaults presumably
>> due to uninitialized members.
>> I understand there may seldom be a reason to use uninitialized
>> objects,
>> so "don't do that," but as I said this was unintentional and it seems
>> that it should have generated a warning, which have saved some
>> head-scratching.
>>
>> Code to reproduce:
>>
>> #include <string>
>> std::string f(std::string &s) {
>>    s.append("x");
>>    return s;
>> }
>> int main() {
>>    std::string a = f(a);
>> }
>>
>> Compile and run (no warning):
>>
>> $ g++ -o uninit_obj uninit_obj.cpp -std=c++23 -Wall -Wpedantic -
>> Wextra
>> && ./uninit_obj
>> Segmentation fault (core dumped)
>>
>> No difference whether using -O0 (or 1 2 3)
>
> As I understand it, -Wmaybe-uninitialized is purely intraprocedural
> i.e. it works within each individual function, without considering the
> interactions *between* functions.

If you compile

#include <string>
static std::string f(std::string &s) {
  s.append("x");
  return s;
}
void g() {
  std::string a = f(a);
}

with -O3, by the time we get to the uninit pass, function g starts with

void g ()
{
   size_type __dnew;
   struct string a;
[...]
   <bb 2> [local count: 1073741824]:
   _26 = a._M_string_length;
   if (_26 == 4611686018427387903)

which should not require any interprocedural logic.

-- 
Marc Glisse